Longitudinal bone growth occurs at the growth plate, a thin layer of cartilage which consists of three principal zones: the resting zone, the proliferative zone, and the hypertrophic zone. Studies in our laboratory indicate that the resting zone contains stem-like cells that are capable of generating new clones of proliferative chondrocytes. These proliferative cells undergo clonal expansion followed by cellular hypertrophy.[unreadable] [unreadable] With age, growth plate chondrocyte proliferation slows down, causing longitudinal bone growth to slow and eventually stop. Our previous studies indirectly suggest that proliferation slows because the stem-like cells of the resting zone have a finite proliferative capacity which is gradually exhausted. To test this hypothesis more directly, we measured the proliferation rate in resting zone chondrocytes using continuous bromodeoxyuridine labelling. Consistent with our hypothesis, we found that the proliferation rate decreases with age as does the number of resting zone chondrocytes per area of growth plate. [unreadable] [unreadable] Glucocorticoid excess slows growth plate senescence. To explain this effect, we hypothesized that glucocorticoid inhibits resting zone chondrocyte proliferation, thus conserving their proliferative capacity. Consistent with this hypothesis, we found that dexamethasone treatment decreases the proliferation rate of resting zone chondrocytes and slows the numerical depletion of these cells. [unreadable] [unreadable] Our findings support the hypotheses that growth plate senescence is caused by qualitative and quantitative depletion of stem-like cells in resting zone and that growth-inhibiting conditions, such as glucocorticoid excess, slow senescence by slowing resting zone chondrocyte proliferation and slowing the numerical depletion of these cells, thereby conserving the proliferative capacity of the growth plate. [unreadable] [unreadable] We have also investigated in detail one particular gene that might be involved in growth plate senescence. p27/Kip1, a cyclin-dependent kinase inhibitor, negatively regulates proliferation of multiple cell types. We therefore assessed the role of p27 in the regulation of growth plate chondrocyte proliferation. p27 mRNA expression was detected by real-time PCR in both proliferative/resting and hypertrophic zones of the mouse growth plate. To determine whether this expression is physiologically important, we studied skeletal growth in 7-wk-old mice lacking a functional p27 gene. In these mice, body length was modestly increased compared to wild-type littermates. In the proximal tibiae, proliferation of growth plate chondrocytes was increased but tibial length was not significantly greater than controls. p27 ablation had no measurable effect on growth plate morphology, including the number of proliferative or hypertrophic chondrocytes. Treatment with dexamethasone in vivo inhibited longitudinal bone growth similarly in p27-deficient mice and controls, indicating that p27 is not required for the inhibitory effects of glucocorticoid on growth plate function. p27-deficient mice had increased width of the femoral diaphysis, suggesting that p27 acts normally to inhibit periosteal bone growth. In summary, our findings suggest that p27 acts a negative regulator of growth plate chondrocyte proliferation. [unreadable] [unreadable] In clinical studies, we have investigated the relationship between skeletal development and pubertal maturation. The overall goal of these studies is to explore the primary mechanism that initiates puberty, which is currently unknown. One possible clue is that pubertal maturation often parallels skeletal maturation. Conditions that delay skeletal maturation also tend to delay the onset of puberty whereas conditions that accelerate skeletal maturation tend to hasten the onset of puberty. To examine this relationship, we previously studied boys with congenital adrenal hyperplasia and familial male-limited precocious puberty, two conditions that accelerate maturational tempo, and boys with idiopathic short stature in which maturational tempo is sometimes delayed. In all three conditions, the onset of central puberty generally occurred at an abnormal chronological age but a normal bone age. Boys with the greatest skeletal advancement began central puberty at the earliest age whereas boys with the greatest skeletal delay began puberty at the latest age. Furthermore, the magnitude of the skeletal advancement or delay matched the magnitude of the pubertal advancement or delay. This synchrony between skeletal maturation and hypothalamic-pituitary-gonadal axis maturation was observed among patients within each condition and also between conditions. In contrast, the maturation of the hypothalamic-pituitary-gonadal axis did not remain synchronous with other maturational processes including increases in weight, height, or body mass index. These data indicate that in boys with abnormal developmental tempo, maturation of the skeleton and the hypothalamic-pituitary-gonadal axis remain synchronous. [unreadable] [unreadable] This synchrony raises the possibility that skeletal maturation influences pubertal onset. To determine whether this concordance is also present in normal children, we analyzed data from 30 normal boys participating in a longitudinal study. Height, weight, and serum testosterone concentrations were assessed every 6 months and bone age every year. Pubertal onset was defined by serum testosterone greater than 30 ng/dL. The variability in bone age at onset of puberty was not less than the variability in chronological age. In addition, there was no significant correlation between skeletal advancement and pubertal advancement. Similarly, there was not a significant correlation between pubertal advancement and height age advancement, weight age advancement, or body mass index age advancement. These findings do not support the hypothesis that skeletal maturation directly influences the age of pubertal onset in normal boys. To explain both these findings in normal boys and the previous findings in boys with abnormal maturation requires a more complex model.